The present invention pertains generally to the field of electronic amplifier circuits for hearing aids, and more specifically to high efficiency circuits which are capable of operating with only an extremely small current drain on the battery, but without any sacrifice in output sound volume.
The major design objective in the hearing aid industry is to provide devices which will restore, as nearly as possible, the ability to hear speech at normal conversational levels to individuals suffering various types of hearing loss, with the least amount of cost and discomfort to the user of the hearing aid. To this end, much effort has gone into reducing the physical size of the aid and in improving the intelligibility of normal speech to a user of the aid. Many recent advances in hearing aid circuit design have unfortunately resulted in an increased number of electronic components, which increases both the physical size and the operating current of the circuit. Circuits designed for increased sound volume output, and also circuits designed for improved intelligibility by advanced signal processing techniques have resulted in larger current drain and shorter battery life.
It is of course desirable to reduce current drain thereby to increase battery life, in order to minimize the battery cost to the user, which in some cases may exceed One Dollar per week. Not only is the economic cost of the replacement batteries important, but the necessity of frequent battery replacement can be a nuisance or an embarrassment to the user. With some prior art hearing aids, the battery must be replaced every few days, and this may lead to anxiety on the part of the user who must continually check to see that replacements are on hand in order to avoid embarrassment should the old battery run down.
The present invention provides an amplifier circuit with such low current drain that it will operate more than three times as long on a given battery as conventional prior art hearing aids which provide comparable output levels. In addition, the circuit used in the present invention may be used with a special feedback circuit to obtain a sharp low frequency roll-off characteristic which is desirable in correcting for certain types of hearing loss.
One type of prior art hearing aid amplifier uses an output transistor operating in Class A, with the receiver connected as the collector load of the transistor. In such a circuit design for a moderately high output (approximately 115 db sound pressure level at saturation with 38 db of acoustic gain), the entire circuit may draw approximately 0.5 milliamperes of current, approximately 80% of which is used in the output stage. Since the output transistor should be biased somewhere near its midpoint in order to avoid clipping and distortion at high output levels, the transistor must be biased so that at its quiescent point, approximately half of the battery voltage is dropped across the output transistor, and the other half across the receiver. In order to avoid drawing excessive quiescent currents, it has been necessary to use a receiver having a relatively high DC resistance, 900 ohms being typical. However, this results in a low efficiency circuit in that when an audio signal is being amplified, a large portion of it is dissipated in the large resistive component of the receiver impedence. Such a circuit will give a life of about 350 to 400 hours on a certain type of mercury battery.
Another type of prior art hearing aid circuit succeeds in reducing the current drain to about 0.25 milliamperes, but only at the expense of reduced maximum sound pressure level output. This type of prior art circuit also uses a Class A output transistor and can achieve approximately 800 hours of operation on the same type of battery, but its maximum output is limited to approximately 108 db sound pressure level at 27 db of acoustic gain. Obviously, this prior art circuit may be useful for persons who do not require a great deal of amplification, but it clearly does not solve the problem of excessive battery drain in higher powered amplifiers.
In contrast, the present invention provides a hearing aid amplifier circuit which is capable of providing greatly reduced current drain, at no sacrifice in maximum output sound pressure level. For example, in one circuit according to the present invention, a maximum average sound pressure level of 116 db (with peaks of 120 db) is achieved, but at a current drain of only 0.150 milliamperes. This gives an average life with the same battery as in the preceding examples of 1300 hours, which is over three times as long as conventional aids in the same power class.